better field targets for improved ECS compatibility

[kares]

The JMS plugin does a few places at the top level - which might lead to ECS conflicts:

• for a MapMessage fields are set as they're obtained without potential control to re-target them, might need to introduce a standalone target option in the input itself
• JMS attributes (headers) are set at top level, user can control which to skip but a better model might be to set these as [@metadata][input][jms][attributes]... and allow to user to copy the relevant ones to a top level field down the pipeline?
• JMS message properties are also set at the top level - we should use the same convention as for attributes in ECS mode

The plugin should also leverage event_factory support when in a need to create events directly.